1. Field of the Invention
This invention relates to an active noise cancellation system, and to a method of active noise cancellation.
2. Description of the Related Art
It is known to provide a feedforward active noise cancellation system, in which one or more microphones are used to generate an electrical signal representative of the ambient noise. This ambient noise signal is passed to a filter, to generate a noise cancellation signal. The noise cancellation signal is applied to a speaker, which is located close to the user's ear, for example in an earphone device or in a handset that is intended to be held close to the ear. By suitable choice of the filter, the effect of the noise cancellation signal is to generate sounds that are equal in magnitude, but opposite in phase, to the ambient noise arriving at the user's ear, so that the sound waves cancel, and the noise that reaches the user's ear is reduced.
One typical use of such an active noise cancellation system is in a sound reproduction device, where the noise cancellation signal is added to a wanted sound signal, for example representing music or speech, before being applied to the speaker.
It is also known to provide an element of feedback, by locating an error microphone close to the entrance to the user's ear canal, and using the signal detected by the error microphone to adapt the filter that is used to generate the noise cancellation signal. In a fully adaptive system, the error signal is used to adapt the frequency response of the filter. In a simpler system, the error signal is used to adapt the gain of the filter, without altering the frequency response.
It is commonly required to use active noise cancellation in systems that are intended to be used outside, where there might be wind. For example, mobile telephone handsets and playback devices are often used in such conditions. However, the effect of conventional active noise cancellation systems in the presence of wind noise is to magnify the wind noise, rather than cancel it. This problem arises because wind is a randomly varying air pressure that is felt more than heard, whereas ambient noise is audible. The ambient noise can be cancelled by an inverted version of the signal generated by the noise microphone. However, although wind is not heard to a great extent, it causes turbulence around the noise microphone, and thus generates a component of the noise signal. A corresponding component of the noise cancellation signal will be generated, and will cause the speaker to produce sounds. However, the inverted version of the wind noise component has no audible counterpart to cancel, and so the wind noise is made more audible by the active noise cancellation system. Even a small gust of wind, that would not itself be very loud or disturbing directly at the ear, can yield an annoying level of wind noise.